Device and method for carrying out heterogeneously catalysed gas phase reactions with heat tonality

ABSTRACT

Apparatus for substantially isothermal operation of a heterogeneously catalyzed gas phase reaction involving a pronounced exotherm comprises at least one reactor space ( 101 ) having an inlet ( 131, 141 ) and an outlet ( 143 ), wherein  
     the reactor space is bounded by heat-removing walls which are spaced apart substantially uniformly at a distance of ≦30 mm along the main flow axis of a reaction gas,  
     the reactor space is fitted with catalyst-coated tapes ( 120, 132 ),  
     the tapes are flexible and pervious to the reaction gas in all spatial directions and have a surface to volume ratio of 50 to 5000 m 2 /m 3  and also a good thermal conductivity,  
     the reaction gas flows through the reactor space at a velocity of ≧200 m 3  per m 2  of frontal area per hour, and  
     a heat exchange medium flows on that side of the reactor wall which is remote from the reactor space.

[0001] This invention relates to a process and apparatus for the substantially isothermal operation of gas phase reactions involving a pronounced exotherm, specifically oxidative dehydrogenations, over a solid catalyst.

[0002] DE-A 42 43 500 discloses the use of specific catalyst-coated knitted wire inserts for exhaust gas cleaning. The layers of knitted or woven wire are thermally and/or mechanically fixed in the wound state. Problems are the complicated construction of the catalyst insert and the poor heat transport within same.

[0003] DE-A 41 09 227 discloses an exhaust gas filter and/or catalyst

[0004] (i) with a feed duct leading to a

[0005] (ii) filter or catalyst body composed of metallic materials of construction, the materials of construction of the filter or catalyst body using compression-molded wires or fibers in random, braided, knitted or woven form or in powder, granule or chip form to form a body that is pervaded by voids and through which the exhaust gas passes, and

[0006] (iii) with an exit duct for the exhaust gas cleaned by the filter or catalyst body.

[0007] The filter or catalyst body can have heat exchanger pipes or ducts passing through it transversely or opposite to the exhaust gas flow direction through the filter or catalyst body.

[0008] EP-B 201 614 describes a reactor for carrying out heterogeneous, catalyzed chemical reactions that contains at least partly corrugated tape-form catalyst bodies whose corrugation is disposed at an inclination to the main flow axis and oppositely directed in adjacent plates, the pitch of the corrugation of the catalyst body being less than the pitch of the adjacent corrugated plates and the surface area of the catalyst being larger than the surface area of an adjacent corrugated plate. The catalyst can be a body which is coated with a catalytically active material and which may be constructed as braided or knitted wire. The complicated corrugation of the plates favors bypass formation, inhibits eddying and thus compromises mass transfer. In addition, the envisioned compact packing element does not provide for effective removal of the heat of reaction.

[0009] EP-B 0 305 203 describes the operation of heterogeneously catalyzed reactions under nonadiabatic conditions. To this end, an annular reactor chamber with heat-transmitting walls is packed with monolithic catalysts in the form of catalyst sheets. The monolithic catalysts have channels which are angled relative to the overall flow direction, so that the reaction fluid is routed at an acute angle from one reactor wall to the other. The shearing stress exerted on the reaction fluid is extremely high (high pressure drop) in reactor wall vicinity and otherwise rather low (poor mass transfer). The reactor is complicated to fabricate, since the pressure drop depends decisively on the geometry between reactor wall and monolithic catalyst.

[0010] EP-B 0 149 456 relates to a process for preparing a glyoxylic ester by oxydehydrogenation of the corresponding glycolic ester in the gas phase using a tubular reactor comprising a catalyst support made of at least one cylindrical monolith having essentially the same diameter as the reactor tube and containing channels from 1 to 10 mm in diameter which lead from the inlet to the outlet of the reactor tube, from 60 to 90% of the volume of the monolith being formed by hollow spaces. The channels can form an angle of from 20 to 70° with the reactor axis. This measure directs the reaction fluid to the reactor walls and thus promotes the removal of the heat of reaction. This process has the same disadvantages as the process known from EP-B 0 305 203.

[0011] DE-A 197 25 378 describes a compact fixed bed reactor for catalytic reactions in the gaseous and/or liquid phase that is transited by two streams of material in co- or countercurrent. The flow channels for the two streams of material are formed by a concertinaed dividing wall. The folds in this dividing wall have been formed into undulating structures in such a way that continuous flow channels are created for the fluid streams. The undulating structures serve both as spacers between opposite folds of the dividing wall and as a catalyst support and ensure improved heat transport to and from the dividing wall. The undulating structures are rigid constructions whose dimensions limit the minimum spacing between the folds of the dividing walls and also the amount of catalyst which can be applied to these undulating structures. The ratio of surface area of the undulating structures (ie. of the catalyst) to heat exchanger volume is not more than 800 m²/m³ on the basis of a maximum industrially feasible fold width of 5 mm and a fold angle of 90°. In addition, the fabrication of the reactor is relatively costly.

[0012] It is an object of the present invention to provide a reactor for operating heterogeneously catalyzed gas phase reactions involving a pronounced exotherm that combines good heat removal and supply at the site of the heterogeneously catalyzed reaction with a good surface to volume ratio for the catalyst.

[0013] We have found that this object is achieved by an apparatus for substantially isothermal operation of a heterogeneously catalyzed gas phase reaction involving a pronounced exotherm, comprising at least one reactor space having an inlet and an outlet, wherein

[0014] the reactor space is bounded by heat-removing walls which are spaced apart substantially uniformly at a distance of ≦30 mm along the main flow axis of a reaction gas,

[0015] the reactor space is fitted with catalyst-coated tapes,

[0016] the tapes are flexible and pervious to the reaction gas in all spatial directions and have a surface to volume ratio of 50 to 5000 m²/m³ and also a good thermal conductivity,

[0017] the reaction gas flows through the reactor space at a velocity of ≧200 m³ per m² of frontal area per hour, and

[0018] a heat exchange medium flows on that side of the reactor wall which is remote from the reactor space.

[0019] The subject apparatus is useful for operating not only substantially exothermic but also substantially endothermic reactions, since it provides for rapid heat removal and heat supply, respectively. Examples of substantially endothermic reactions include oxidative dehydrogenations such as that of 3-methyl-3-buten-1-ol, while examples of substantially exothermic reactions include the hydrogenation of double or triple bonds and also aromatics such as the hydrogenation of benzene to cyclohexane. The enthalpies of the reactions are for example in the range from 30 to 75 kcal/mol.

[0020] The subject apparatus also makes it possible to operate under reduced pressure or elevated pressure as well as atmospheric pressure, ie. at pressures from 1·10⁻³ to 100 bar, especially from 0.5 to 40 bar. The subject apparatus can thus be used over a wide pressure range.

[0021] Reaction gas for the purposes of the present invention denotes the mixture of gaseous reactants and optionally added further gaseous substances that do not react with the reactants under the reaction conditions. The heat exchange medium can be a liquid, a gas or a molten salt bath, depending on the desired temperature. When the heat exchange medium is used for absorbing and removing heat, it is also known as cooling fluid. Temperatures from −20° C. to 400° C. can be efficiently realised. The rapid heat removal or heat supply made possible by the subject apparatus provides very accurate heat control. It is possible, for example, to set a temperature of 370° C.±10° C., especially ±5° C. In contradistinction to traditional fixed bed reactors, there are no temperature spikes with the use of the subject apparatus.

[0022] The reactor space can be not only annular but also cylindrical, rectangular or square.

[0023] The subject apparatus is easily realizable by fitting the catalyst-coated tapes (catalyst tapes) into the gap of a commercially available heat exchanger. Thus, the reactor tube does not conform to the catalyst, but the catalyst tapes are conformed to the reaction space. Any desired heat exchanger can be used. Not only annular gap heat exchangers but also plate type heat exchangers or spiral type heat exchangers are useful. Examples of heat exchangers include designs as described in ISO 15547 or in W. R. A. Vauck, H. A. Müller, Grundoperationen chemischer Verfahrenstechnik, Verlag Theodor Steinkopff Dresden 1974, 4^(th) edition, pages 438-440, or in the MB1 section of the VDI Wärmeatlas, VDI Verlag, 3^(rd) edition, 1977 (to realize the heat transfers described in the CB3 section). The wall spacing and hence the gap width or gap diameter of the heat exchangers used is preferably in the range from 0.5 to 30 mm, especially in the range from 1 to 20 mm, in particular in the range from 1.5 to 10 mm or from 1.8 to 5 mm.

[0024] When annular gap heat exchangers are used, the catalyst tapes are installed in the reactor space formed by two coaxial tubes and are cooled (or respectively heated) through the wall of the inner tube and/or of the outer tube. This apparatus according to the invention is also known as an annular gap heat exchanger reactor. Plate type heat exchangers have a square or rectangular reactor space which is optionally subdivided by additional heat-conducting walls which force the reaction gas to take a zigzag course through the reactor space. A plate type heat exchanger reactor according to the invention is obtained by installing catalyst tapes in the reactor space, if necessary without catalyst tapes being used where a change of direction is greatest in order that an excessively large pressure drop may be avoided.

[0025] A subject apparatus that utilizes a spiral type heat exchanger (“spiral type heat exchanger reactor”) has a particularly cylindrical reactor space which is packed very uniformly with catalyst tapes.

[0026] Catalyst tapes are sheetlike, smooth constructions, which can be formed as wovens, loop-drawn knits, loop-formed knits, perforated plates or—in the case of metal as the material of construction—as a rib mesh.

[0027] It may also be possible to use felts, films or foils, but these have to be combined with wovens, loop-drawn knits, loop-formed knits, perforated plates or rib meshes in such a way that the felts, films or foils have to be oriented parallel to the main flow direction and the wovens, loop-drawn knits, loop-formed knits, perforated plates or rib meshes serve as spacers for the felts, films or foils. It is also possible for felts, films or foils oriented parallel to the main flow direction to be alternated with wovens, loop-drawn knits, loop-formed knits, perforated plates or rib meshes when installed in the reactor space. Preference is given to using wovens, loop-drawn knits or loop-formed knits.

[0028] Catalyst tapes are flexible, ie. bendable and extendible, in all spatial directions. They are accordingly unstructured catalyst articles which are readily conformable to the dimensions of the reactor space, especially the gaps of commercial heat exchangers. Their use does not require any fixation or orientation with regard to the main flow axis. Since catalyst tapes are flexible in all spatial directions, they become fixed automatically. In general, the catalyst tapes are introduced individually, curled or in layers into the reactor space without prior deformation (for example due to embossing of the surface structure such as corrugations by means of a tooth wheel roll). This permits a higher packing density for the catalyst tapes coupled with uniform filling of the reactor space and maximum suppression of undesirable bypass formation, which is effected in an increased mass transfer. The catalyst tapes are installed by manually laying, standing or pushing them into the gap of the heat exchanger. Limiting factors are the dimensions of the reactor space and the thickness of the catalyst tapes. Not only one catalyst tape can be installed but a plurality. The catalyst tapes can be positioned not only distributed over the entire reactor space of the heat exchanger but also only in sections selected by one skilled in the art. Since the catalyst tapes are flexible in all spatial directions, they can not only be extended but also layered, folded or curled. By extending is meant the lengthwise or widthwise stretching of a catalyst tape. Whereas, for example, corrugated sheets cannot be extended, catalyst tapes can be extended by up to 60%, depending on their material of construction. By layering is meant the superposing of at least two catalyst tapes, and folding is to be understood as meaning the superposing of one and the same catalyst tape with the direction of the tape changing by 180° in certain or arbitrarily selected sections. The layered catalyst tapes may optionally be further folded or curled.

[0029] The surface area of the catalyst tapes may be increased by more pronounced folding or curling of the catalyst tapes without substantially increasing the volume of these more substantially folded or curled catalyst tapes. The catalyst tapes have a high surface to volume ratio in the range from 50 to 5000 m²/m³. Such a high surface to volume ratio cannot be achieved with catalyst monoliths or dumpable catalyst material, nor such a high scope for variation in the adjustment of this surface to volume ratio. For example, such a high scope for variation is not achievable with the structured spacers described in DE-A 197 25 378. The catalyst tapes, moreover, are pervious to the reaction gas and—compared with structured catalyst articles such as monoliths or dumpable material—have a good heat transfer coefficient (see VDI Wärmeatlas, VDI Verlag, 3^(rd) edition, 1977, CB3 section) and hence good thermal conductivity, so that the heat of reaction is rapidly transferred by the catalyst tapes to the reaction walls and vice versa. Another factor promoting rapid heat transport is the small wall spacings in the reactor space, which are generally ≦30 mm, preferably ≦20 mm, particularly preferably ≦10 mm. The volume of the reactor space is predetermined by the volume of the gaps of commercial heat exchangers.

[0030] The catalyst tapes, moreover, are mechanically very stable, so that the heterogeneously catalyzed gas phase reactions can be operated even at high flow velocities for the reaction gas without significant attrition of the catalyst. The subject apparatus can be used at low flow velocities, but it is superior to conventional reactors with catalyst monoliths or dumpable catalyst material especially at flow velocities ≧200 $\frac{m^{3}}{{frontal}\quad {{{area}\quad\left\lbrack m^{2} \right\rbrack} \cdot h}},$

[0031] especially at flow velocities ≧300 $\frac{m^{3}}{{frontal}\quad {{{area}\quad\left\lbrack m^{2} \right\rbrack} \cdot h}},$

[0032] in particular at flow velocities ≧1000 $\frac{m^{3}}{{frontal}\quad {{{area}\quad\left\lbrack m^{2} \right\rbrack} \cdot h}}.$

[0033] The flow velocity is chosen according to process (operation at reduced pressure, atmospheric pressure or elevated pressure) and as a function of the ratio of catalyst tapes volume to reactor space volume. Gas flow velocities up to 70 m/s can be realized in the apparatus of the invention before the catalyst tapes have been installed. Typical values of gas flow velocities in heat exchangers are 40 m/s. When the apparatus according to the invention has been packed with catalyst tapes, it can be operated at flow velocities of from 200 to 15000 $\frac{m^{3}}{{frontal}\quad {{{area}\quad\left\lbrack m^{2} \right\rbrack} \cdot h}},$

[0034] especially at flow velocities of from 300 to 15000 $\frac{m^{3}}{{frontal}\quad {{{area}\quad\left\lbrack m^{2} \right\rbrack} \cdot h}},$

[0035] in particular at flow velocities of from 1000 to 15000 $\frac{m^{3}}{{frontal}\quad {{{area}\quad\left\lbrack m^{2} \right\rbrack} \cdot h}}.$

[0036] The stated velocities are superficial velocities determined using a gas meter.

[0037] Such high flow velocities are not realizable with dumpable catalyst material, not only on account of the attrition, but also on account of the associated high pressure drop. Since, in the apparatuses according to the invention, significant pressure drop can be avoided by choosing a suitable flow velocity, no compressors are needed in this case either to compensate the pressure drop, so that use of the subject apparatus provides for additional cost saving over the use of conventional reactors.

[0038] Owing to their mechanical stability, the catalyst tapes are simple to remove from the reactor spaces and to exchange without the problems of removing the fine catalyst attritus which are associated with dumpable catalyst material. It is astonishing that, when such unstructured catalyst tapes are used, the selectivity of the heterogeneously catalyzed reactions in the gas phase is retained or even improved by the rapid heat transport.

[0039] The subject apparatus, moreover, is designed for maintaining a high but uniform shearing stress on the reaction gas. First, as mentioned above, it will withstand a high cross-sectional flow velocity without attrition of the catalyst. Secondly, the reaction gas is exposed to a uniformly high shearing stress in the reactor space fitted with catalyst tapes. This provides for uniform mixing of the reaction gas and hence for a constant degree of dispersion of the reaction gas as it passes through the reactor space. The high flow velocities and the efficient mixing of the reaction gas mean that the apparatuses according to the invention provide similar conversions to conventional reactors even though—compared with the operation of the reactions in conventional reactors—the operation of the reactions in the apparatuses according to the invention has lower catalyst requirements. A further advantage of the subject apparatus is that there is no need for costly structuring of the catalyst or catalyst support, so that it is again possible to save costs.

[0040] The catalyst tapes generally have a fine structure. In the case of wovens and loop-drawn knits, the fine structure resides in the rectangles formed by the wire or thread which each share the sides with one another. In fact, it is preferable for the angle of pitch, formed by one side of the two sides of a rectangle with the main flow axis of the reaction gas, to be randomly distributed. By ‘randomly distributed angle of pitch’ is meant that the catalyst tapes are introduced into the reactor space in such a way that ideally all possible angles of pitch are actualized and ideally a chaotic meshwork is formed as a result. In such a chaotic meshwork, the sequence of voids, wires and threads in the reactor space is ruleless as a consequence of the random orientation of the catalyst tapes. This minimizes bypass formation within the reactor and maximizes heat and mass transfer as a consequence of a turbulent flow regime.

[0041] The materials of construction used for the support are selected from the group consisting of metallic, ceramic and plastics materials of construction in line with the deformations occurring in the course of production, reshaping and use. Useful metallic, ceramic and plastics materials of construction generally form fibrous structures. Examples of such metallic materials of construction are pure metals such as iron, copper, nickel, silver, aluminum and titanium or alloys such as steels, for example nickel, chromium and/or molybdenum steel, brass, phosphorus bronze, Monel and/or nickel silver. Examples of ceramic materials of construction are alumina, silica (glass fibres) zirconia and/or carbon. Examples of plastics are polyamides, polyethers, polyvinyl, polyethylene, polypropylene, polytetrafluoroethylene, polyketones, polyether sulfones, epoxy resins, alkyd resins, urea and/or melamine resins. Preference is given to metals, asbestos substitutes, glass fibers, carbon fibers and/or plastics, especially metals, ie. pure metals and alloys, since these have a very good heat transfer coefficient. Very particular preference is given to inexpensive stainless steels which are given an appropriate catalytic coating.

[0042] The tapes coated with catalyst according to the invention are in particular woven or loop-drawingly knitted metal fabrics. For the purposes of the present invention, loop-drawingly knitted metal fabrics are metal fabrics formed from one continuous metal thread. Woven metal fabrics, in contrast, are fabrics formed from at least two metal threads. The wire diameter is generally in the range from 0.01 to 5.0 mm, preferably from 0.04 to 1.0 mm, in the case of woven or loop-drawingly knitted metal fabrics. The mesh size can be varied within wide limits. The catalyst tapes can be produced by the process described in U.S. Pat. No. 4,686,202 and EP-B 0 965 384.

[0043] Catalyst tapes embodied as woven metal fabrics can further be coated by the process described in EP-B 0 564 830. EP-B 0 564 830 does not expressly describe the coating of loop-drawingly knitted metal fabrics with catalyst, but they shall be treated in the same way as woven metal fabrics. The coating of woven or loop-drawingly knitted metal fabrics with catalysts may also be effected by conventional dip processes, for example according to the process described in EP-A 0 056 435.

[0044] The disclosures of U.S. Pat. No. 4,686,202, EP-B 0 965 384, EP-B 0 564 830 and EP-A 0 056 435 are hereby fully incorporated by reference.

[0045] When the metal forming the woven or loop-drawingly knitted metal fabric is itself catalytically active (possibly after a treatment), coating may be dispensed with entirely.

[0046] The invention will now be more particularly described with reference to FIGS. 1 to 3.

[0047]FIG. 1 shows a schematic drawing of a plate type heat exchanger reactor according to the invention,

[0048]FIG. 2 shows a side view of the interior of a spiral type heat exchanger reactor,

[0049]FIG. 3 shows a further side view of a spiral type heat exchanger reactor.

[0050]FIG. 1 shows an inventive plate type heat exchanger reactor (101). The catalyst-coated tapes bear the reference numeral 120. 131 designates the feed for the reaction gas into the reactor space and 143 its exit line. The feed and exit lines for the cooling fluid bear the reference numerals 144 and 142 respectively.

[0051]FIG. 2 shows a side view of a spiral type heat exchanger reactor according to the invention. 131 identifies the feed for the reaction gas into the reactor space (reactor inlet). 132 identifies the reactor passage which will receive the catalyst-coated tapes, which will take up the entire space in more or less dense packing. 133 identifies the cooling passage, which is to receive the cooling fluid.

[0052]FIG. 3 is a side view of a spiral type heat exchanger reactor and identifies the arrangement of the feed and discharge stubs. 141: reaction gas feed (reactor inlet), 142: cooling fluid discharge, 143: reaction gas discharge (reactor outlet), 144: cooling fluid feed. Reaction gas and cooling fluid are here arranged in countercurrent in order that heat transfer may be maximized. If the amount of heat released at the reactor inlet specifically is critical with regard to, for example, selectivity and catalyst stability, then a cocurrent arrangement is advisable.

[0053] The example hereinbelow illustrates the invention.

INVENTIVE EXAMPLE

[0054] Oxidation of 3-methyl-3-buten-1-ol to 3-methyl-2-butenal in the Gas Phase as per Equation I

[0055] The reaction is conducted over a silver catalyst. The catalyst according to the invention is prepared by coating a woven tape of heat-resistant stainless steel, material No. 1.4764 (as per Stahl-Eisenliste, 8th edition, published by: Verein Deutscher Eisenhüttenleute), with silver in an electron beam vapor deposition unit. This coating technique was used to coat the woven metal tape on both sides with 300 nm of Ag. 50 cm² of this woven catalyst tape were introduced in a double layer—without deformation—into an annular gap heat exchanger reactor 2 mm in width. The amount of active component was 34 mg of silver. To oxidatively dehydrogenate 3-methyl-3-buten-1-ol (MBE), a mixture of 85% by weight of MBE and 15% by weight of H₂O was vaporized at 150° C., mixed with preheated air and superheated by means of a preheater to an inlet temperature of 370° C.

[0056] After leaving the annular gap, the gaseous reaction product was cooled with cooling brine to 0° C., and the condensate was collected in a cooled separator. The gas fraction of the reaction product passed through dry ice (to condense the low boiler fractions) to a gas chromatography analyzer and thereafter by a gas meter into the waste gas. The combined quantities of condensate were separated into an organic and an aqueous phase. Both phases were analyzed. The result obtained was a selectivity of 83% from a conversion of 54%.

Comparative Example

[0057] Instead of the annular gap heat exchanger reactor in the inventive example, a fixed bed reactor with a 30 mm deep layer of silver granules conforming to DE-A 27 15 209 is installed into the same plant and the conversion of MBE is carried out similarly to the inventive example. The results are summarized in the table which follows: Superficial MBE Superficial velocity air velocity Conversion Selectivity Catalyst Reactor [g/cm² h] [l/cm² h] [%] [%] Woven tape coated with Annular gap heat exchanger 279 94 54 83 300 nm layer of reactor, 2 mm Ag (34 mg of Ag in total) passage width Ag granules, Fixed bed reactor,  69 27 54 73 17 g of Ag, about 4.5 cm³ 30 mm bed height

[0058] It can be seen that, for the same conversion, the selectivity of the comparative example is 10% worse than that of the inventive example.

[0059] In addition, the inventive example is more economical, since only 0.034 g of silver had to be used instead of 17 g of silver. Another economic plus is that the ability to employ higher flow velocities makes it possible to obtain a higher conversion per hour. 

We claim:
 1. Apparatus for substantially isothermal operation of a heterogeneously catalyzed gas phase reaction involving a pronounced exotherm, comprising at least one reactor space (101) having an inlet (131, 141) and an outlet (143), wherein the reactor space is bounded by heat-removing walls which are spaced apart substantially uniformly at a distance of ≦30 mm along the main flow axis of a reaction gas, the reactor space is fitted with catalyst-coated tapes (120, 132), the tapes are flexible and pervious to the reaction gas in all spatial directions and have a surface to volume ratio of 50 to 5000 m²/m³ and also a good thermal conductivity, the reaction gas flows through the reactor space at a velocity of ≧200 m³ per m² of frontal area per hour, and a heat exchange medium flows on that side of the reactor wall which is remote from the reactor space.
 2. Apparatus as claimed in claim 1, wherein the reactor space is formed by the gap of a heat exchanger.
 3. Apparatus as claimed in claim 1 or 2, wherein the reactor space is formed by the gap of a spiral type, plate type or annular gap heat exchanger.
 4. Apparatus as claimed in any of claims 1 to 3, wherein the tapes are formed from metals, asbestos substitutes, glass fibers, carbon fibers and/or plastics.
 5. Apparatus as claimed in claim 4, wherein the tapes (120, 132) are formed by a woven metal fabric or by a loop-drawingly knitted metal fabric.
 6. Apparatus as claimed in any of claims 1 to 5, wherein the sequence of voids, wires or threads in the reactor space is ruleless as a consequence of the random orientation of the tapes (120, 132) with regard to the main flow axis of the reaction gas.
 7. Apparatus as claimed in any of claims 1 to 6, wherein the walls in the reactor space are spaced from 0.5 to 30 mm, preferably from 1 to 20 mm, particularly preferably from 1.5 to 10 mm, apart.
 8. The use of apparatus as claimed in any of claims 1 to 7 in a process for oxidizing alcohols to aldehydes in the gas phase.
 9. The use of apparatus as claimed in any of claims 1 to 7 in a process for oxidizing 3-methyl-3-buten-1-ol to 3-methyl-2-butenal in the gas phase as per equation I 